1. Field of the Invention
The invention relates to temperature-stabilized sub-1-V bandgap reference voltage circuits, and more particularly to a circuit using a low supply voltage with a plurality of temperature sensors. Programmable adjustable resistors are employed to calibrate the slope (span) and the absolute temperature.
2. Description of the Related Art
Temperature sensors with high precision based on bandgap reference (BGR) circuits which operate in the sub-1-V bandgap reference region using a low voltage power supply are necessary in many applications. Particularly desirable are circuits with multiple temperature sensors where the temperature sensors track each other very closely without requiring complex and extensive circuits placing undue demands on expensive chip real estate and where the programming expense for tracking of the alignment of the temperature sensors is kept to a minimal. The front end of such a circuit demands a low voltage reference. However, it is difficult to approach a stable operation in conventional bandgap reference (BGR) circuits when the supply voltage is under 1.5 V.
Such a circuit is known from the above referenced Related Patent Application, which is expressly referenced herewith. In particular, this circuit produces positive and negative reference voltages based on the area ratio of 1:M of two diode type devices or diode-connected transistors and the ratio of two resistive means. Secondly, these two reference voltages are driving a summing circuit, each using current sources and resistive means to generate a current which is dependent on the ratio of the positive reference voltage and a resistive means, and the ratio of the negative reference voltage and another resistive means. These currents are then summed using a final resistive means which produces the fractional temperature-independent sub-bandgap reference voltage. The magnitude of the fractional, temperature independent sub-bandgap reference voltage is determined by selecting a specific value for that final resistive means.
A review U.S. Patents and U.S. Patent Applications relating to temperature sensors and/or calibration of temperature sensors has yielded:
U.S. Patent Application Publication 2004/0227651 (Furuichi) discloses an A/D-conversion circuit, a temperature-sensor circuit, and an integrated circuit, and a method of adjusting the temperature sensor circuit. The invention comprises a reference voltage generation circuit, a fuse circuit (the adjustment circuit), a current generation circuit, a current/voltage conversion circuit and the aforementioned A/D-conversion circuit. The latter further comprises a voltage generation circuit, other digital circuits and the digital output. The proposed array of analog and digital circuits converts an analog signal into digital data where the conversion voltage is not sensitive to noise. This is a complex and expensive circuit using a lot of chip real estate, but does not provide programmable calibration.U.S. Pat. No. 6,828,847 (Marinca) describes a bandgap reference circuit for producing a stable temperature corrected voltage reference suitable for CMOS fabrication. The circuit produces a voltage reference which is entirely temperature stable over a reasonable temperature range by correcting the temperature curvature of the CTAT base-emitter voltage. However the circuit is not capable of providing multiple temperature outputs with programmable calibration.U.S. Pat. No. 6,824,307 (Vail et al.) teaches the use of a capacitor in series with a circuit element as temperature sensor and a controller, where the circuit element has a resistance that varies with temperature, such as a thermistor. The controller is for charging/discharging the capacitor through the circuit element, measuring the charging/discharging time to a predetermined threshold and determining a temperature based upon the charging/discharging time. This circuit is essentially an RC circuit which may also include a calibration resistor for higher accuracy.U.S. Pat. No. 6,377,110 (Cooper) presents a temperature sensing circuit which employs a silicon diode to provide a highly linear temperature dependent voltage, along with subsequent signal processing stages, either mixed-mode or digital, that collectively implement, through two-point calibration, independent adjustment of slope and output offsets. This circuit provides calibration but no programming capability for multiple temperature point outputs.
It should be noted that none of the above-cited examples of the related art satisfies the requirement of multiple temperature sensors combined with easy and efficient programming for close tracking and with a low demand on valuable chip real estate. The invention described hereafter answers these requirements.